1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a turbine interstage gap between a blade outer air seal and an endwall of an adjacent stator vane.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine, such as an industrial gas turbine (IGT) engine, includes a turbine with multiple rows or stages or stator vanes that guide a high temperature gas flow through adjacent rotors of rotor blades to produce mechanical power and drive a bypass fan, in the case of an aero engine, or an electric generator, in the case of an IGT. In both cases, the turbine is also used to drive the compressor.
It is well known that the efficiency of the engine can be increased by passing a higher temperature gas flow into the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine parts, such as the first stage guide vanes and rotor blades. Also, the turbine inlet temperature is limited to an amount of cooling that can be produced on a turbine vane or blade. Improved cooling capability will also allow for the turbine airfoils to be exposed to higher temperatures. Improved cooling will also allow for longer part life which results in longer engine run times or longer periods between engine breakdowns.
Another problem with the turbines is hot flow ingestion into a section of the turbine that is sensitive to the high temperatures such as the rim cavities or interstage gaps. Bow wave driven hot gas flow ingestion is created when the hot gas core flow enters a vane row where a leading edge of the vane induces a local blockage and thus creates a circumferential pressure variation at an intersection of the airfoil leading edge location of the vane. The leading edge of a turbine vane generates upstream pressure variations which can lead to hot gas ingress into the front gap. If proper cooling or design measures are not undertaken to prevent this hot gas ingress, exposure to the hot gas can result in severe damage to the front edges of the vane endwall as well as the turbine components located upstream of the endwall. FIG. 1 shows a prior art turbine vane with a bow wave effect located upstream of the turbine vanes. The high pressure upstream of the vane leading edge is greater than the pressure inside the cavity formed by the gap. As a result of the pressure differential, the hot gas will flow radially inward into the cavity. The ingested hot gas flows through the gap circumferentially inside the cavity towards the lower pressure zones. The hot gas then flows out at locations where the cavity pressure is higher than the local hot gas pressure.
FIG. 2 shows a prior art turbine with a first stage rotor blade located upstream from a row of second stage stator vanes. An interstage gap is formed between a blade ring for the rotor blade and a blade ring for the stator vane. This arrangement in FIG. 2 includes a rotor blade 27 with a tip that forms a seal with a blade outer air seal (or BOAS) 24, the BOAS 24 is supported by hooks on an isolation ring 22 on a forward side and a blade ring 21 on an isolation ring 25 on the aft side. A first blade ring 21 supports both isolation rings 22 and 25 and includes a cooling air passage that delivers cooling air to an impingement plate 23 that includes impingement holes 28 that discharge jets of impingement cooling air onto a top surface of the BOAS.
An adjacent stator vane assembly includes a second blade ring 26 that supports a guide vane 11 with an outer endwall 12. an interstage gap 29 is formed between the isolation ring 25 and the vane outer diameter endwall 12 in which the hot gas ingress can occur due to the pressure differential described above.
In general, the size of the bow wave is a strong function of the vane leading edge diameter and distance of the vane leading edge to the endwall edge. The pressure variation in the tangential direction with the gap is sinusoidal. The amount of hot gas flow penetrating the axial gap increases linearly with the increasing axial gap width. It is therefore necessary to reduce the axial gap width to a minimum allowable by tolerance limits in order to reduce the hot gas ingress.
As a result of the design of FIG. 2, hot gas flows in and out along the inter-stage gaps and an over-temperature occurs at the blade outer air seal edges and the blade isolation ring corresponding to the hot gas injection location. This over-temperature issue is more pronounced when an insufficient amount of inter-stage gap purge air for the axial gap is available when a strong bow wave is induced by the low solidity vane airfoil creates a high circumferential pressure variation which acts to push the mainstream hot gas into the inter-stage gap 29.